Windows provided with a multilayer film having a low-E (low-E means “low emissivity”, and consequently a low emissivity or high reflection of waves in the infrared spectrum) serve in particular to improve the thermal insulation of windows in buildings and vehicles. The actual thermal insulation function is provided in this case, most of the time, by one or more silver layers (called “functional layer(s)”. In the case of insulating window panes for example, radiative exchange between panes may be almost completely blocked by using windows of the double-glazing type having an emissivity E≦0.1 on the face turned toward the space between the two glass panes. It is thus possible to manufacture insulating window panes having a k value of 1.1 W/m2K.
Window panes provided with optimized low-E multilayer films must also have an overall energy transparency that is as high as possible (the highest possible g value) in order to be able to use the incident solar energy for the energy budget of a building. Finally, the transmission must be as high as possible in the visible range of the spectrum. The color in reflection of coated window panes must be of neutral color, as is the case for conventional insulating window panes.
In certain cases, it is necessary or appropriate to subject windows (at least one of the glass panes of which is it composed) provided with a low-E multilayer film of this kind to a curving or prestressing treatment, of the bending or toughening type. For this purpose, the glass panes are heated before the actual curving or prestressing treatment to a temperature of about 550° C. to 650° C. During this thermal stressing, the silver layer often undergoes structural modifications because of oxidation and diffusion processes. These modifications of the silver layer are manifested, even if they cannot be seen by the naked eye, by a deterioration in the energy values, in particular the transmission and emissivity values. When, for example, insulating window panes made from two glass panes 4 mm in thickness with an argon-filled intermediate space 16 mm in thickness have to have a k value of 1.1 W/m2K, the emissivity of the multilayer system of the glass panes must at most be only 5%. This corresponds to an electrical resistance of at most 4.5 ohms per unit area.
There is a strong demand for multilayer films of this kind, which are capable of withstanding high thermal stresses, the emissivity and the diffuse dispersion (haze) of which are very low; even after a heat (prestressing) treatment, with the optical properties preserved, without the other quality criteria of the film, such as hardness, color and corrosion resistance, being disturbed thereby.
Various suggestions have already been made for the purpose of improving multilayer films of this kind. The desired optical and energy values of coated window panes must therefore be more or less maintained, even in the case of window panes made of glass which, after having been coated with layers, are subjected to a curving or prestressing operation. Deterioration in the properties of the layers as a result of the heat treatment must be prevented or at the very least limited.
Document DE-A1-196 32 788 discloses a prestressable multilayer film of the kind mentioned above, for which the antireflection dielectric layers are composed of an oxide of the metals Sn, Zn, Ti, Si or Bi, or of SiN or AlN, and the sacrificial metal layer of an AlMgMn alloy. The sacrificial metal layer has a thickness of 5 to 10 nm. The term “prestressable” refers in this case to multilayer films of this kind which withstand the high temperatures of the bending and/or toughening operations without significant degradation.
Document DE-A1-196 40 800 describes a silver-based multilayer film having, between the sacrificial metal layer and the upper antireflection layer, an interlayer made of an oxide, a nitride or an oxynitride of the metal of the sacrificial metal layer. The upper antireflection treatment layer is composed of an oxide, a nitride or an oxynitride of a metal other than the metal of the sacrificial metal layer. It may also be a superposition of at least two layers of this type. The term “upper” means that the layer or layers in question lies or lie above the functional layer or at least one of the functional layers of the film, as opposed to the term “lower”.
Documents EP-B1-0 567 735, EP-B1-0 717 014, EP-B1-0 771 766, EP-B1-0 646 551 and EP-A2-0 796 825 disclose prestressable multilayer films having a silver layer as functional layer, with two antireflection layers being each time composed of Si3N4, and the sacrificial metal layer of Ni or NiCr.
The prestressable multilayer film described in document EP-B1-0 883 584 has antireflection layers preferably made of Si3N4, the sacrificial metal layer being composed in this case, however, of silicon.
A prestressable multilayer film, known from document DE-A1-198 50 023, is characterized in that an NiCrOx suboxidized layer having a thickness of between 0.1 et 0.2 nm is embedded in the surrounded silver layer placed between a lower sacrificial metal layer and an upper sacrificial metal layer. The sacrificial metal layers are composed of suboxidized NiCrOx or of suboxidized NiCrOx and suboxidized TiO2. This known multilayer film must allow coated glass window panes to be prestressed and curved without thereby appreciably modifying the optical properties of the multilayer film. The diffuse dispersion (haze) after the heat treatment must in particular be less than 5%.
Document DE-C1-198 52 358 describes a low-E multilayer film capable of withstanding high thermal stresses, having a glass/MeO/ZnO/Zn/Ag/AlMe/MeO/ZnxMeyOn sequence of layers. The AlMe sacrificial metal layer is in this case, as a constituent of the alloy above the silver layer, an alloy of aluminum with one or more of the elements Si, Mg, Mn, Cu and Zn.
None of the known multilayer films fulfills all of the essential properties in an optimal manner. In most cases, after the heat treatment, the multilayer films have too high an emissivity and a relatively high diffuse dispersion (haze).
The objective of the invention is to further improve a low-E multilayer film capable of withstanding high thermal stresses, as mentioned above.